At the present moment, there has been remarkable growth in the memory capacity of optical discs. In particular, Blu-ray discs, which are recorded and reproduced using a blue laser, can achieve tremendously large memory capacities on the order of 25 G bytes for single layered and 50 G bytes for double layered discs, allowing more than four hours of high-quality, high-definition video to be recorded. This increase in capacity per disc is expected to affect not only video content but also music, games, and various other applications as well.
As the memory capacity increases, however, the value of the copyrighted material that is contained on the disc increases as well. Copyright protection of the digital copyrighted material that is recorded is one matter that becomes an issue here.
The conventional approach for copyrighted material that is stored on an optical disc is to record the copyrighted material in an encoded form in order to preclude illegal use of the copyrighted material. Such encoding technology, however, is applicable only when the key remains secret, and the copyright of the copyrighted material cannot be protected once the key information is leaked. The key information thus must be strictly guarded.
With optical discs, the encoded copyrighted material and the key information are recorded onto the disc together. It was thus possible to make illegal copies of the copyrighted material by making an analog copy of the reproduced signal, which is obtained by reproducing the optical disc, by copying it to a master disc as is, or by stripping off, for example, the protective film to expose the recording surface and then copying that recording surface.
At the present time, such illegal copying techniques have hurt the distribution of ordinary copyrighted material on CD and DVD, resulting in significant loss.
One method that has been proposed in light of these issues is the method of trimming a part of the disc reflective layer by irradiating a laser so as to mark each disc individually, and then determining whether or not a disc is a normal disc or an illegally copied disc by confirming the location of this mark (for example, Patent Citation 1). This method allows a mark to be added as separate information to each disc individually, even read-only optical discs that are fabricated by stamping. Although the marks and space columns that are recorded as primary information in a track of the optical disc can be copied to a master disk based on the reproduction signals that are reproduced by tracking the marks and spaces as they are, after disc molding it is extremely difficult to produce a mark by trimming the reflective film at the same position as the optical disc serving as the copy source, and normally there are discrepancies with that marking on the copy source. The present invention allows shifting in the position of the marking to be detected in order to determine whether the disk is a normal disc or an illegal disc.
Another technology that has been proposed involves utilizing changes in the refractive index, changes in the extinction coefficient, changes in the light transmissibility, or changes in the reflectance of a light-transmissible recording member, such as the substrate or protective film, of the optical disc in order to write distinct information to the disc after molding (for example, see Patent Citation 2). This approach involves providing a marking based on changes in the optical constant of the light-transmissible substrate or the protective film when irradiated by an electron beam or ultraviolet radiation. The reproducing device can detect this mark by confirming a sudden change in the reflected light when the reflective film of the disc is irradiated with the reproducing laser. In other words, the mark can be detected as an intensity of the reflected light of the reproducing laser because the optical properties of the light-transmissible substrate are changed. Further, providing the marking by action on the light-transmissible substrate makes the disc resistant against the illegal act of stripping off and copying its recording face.
Another approach that has been proposed involves forming the reflective film of a write-once optical disc as the reflective film of a read-only disc, and then, after disc molding, recording disc-specific information such as key information with a write-once optical disc recording device (for example, Patent Citation 3). With write-once optical discs, the change in the reflectance of the recording film is utilized to record information by irradiating a recording laser. Thus, such discs are resistant against illegal copying in which the protective film is stripped off and then the recording layer is exposed and copied. An added benefit is that because conventional write-once optical disc recording devices can be used, a special device for recording disc-specific information is not required.
Patent Citation 1: International Patent Publication Pamphlet No. 96/016401
Patent Citation 2: Japanese Laid-Open Patent Publication No. 2002-15468
Patent Citation 3: Japanese Laid-Open Patent Publication No. 2000-113452